The electrostatic discharge sensitivity of MOS devices has been well known for the past 30 years. Electrostatic discharge currents may reach the pins during handling of the IC as a result of discharge of charged objects, e.g. through a person, through metal tools, through a case, etc. In order to prevent the electrostatic destruction of the gate insulator film of a MOS element various protection circuits have been proposed. A protective circuit typically is connected between an internal circuit and a bonding pad to which a signal is applied. The protective circuit blunts the abnormal input energy by.
U.S. Pat. No. 4,691,217 entitled "Semiconductor Integrated Circuit Device", issued to Tatsuaki Ueno et al. on Sep. 1, 1987 discloses a semiconductor integrated circuit device protective circuit which includes a MOSFET connected as a diode and a resistor.
An article entitled "A Synthesis Of ESD Input Protection Scheme" by Charvaka Duvvury et al., published in EOS/ESD Symposium Proceedings 1991, describes input protection for CMOS technologies consisting of primary and secondary protection devices. A protection arrangement for integrated circuits (ICs) utilizes an n-channel MOS field-effect transistor (transistor) which is designed to protect the inputs or the outputs of the IC against electrostatic discharge overloading. In order to accomplish this protection, the transistor which is connected to the pin via the bonding pad, i.e., to the respective input or output of the IC, and which always contains a parasitic bipolar transistor, is changed to the bipolar state, in which it can be operated with a high current at a lower voltage. This operation in the bipolar state is referred to as "snapback". Because of the voltage reduction in the bipolar mode, the transistor can absorb more power. To provide further protection for the transistor, the change to the bipolar state should be as fast as possible and the bipolar state should be kept as stable as possible, since a second "snapback", a "second breakdown" of the bipolar transistor, would destroy the latter.
It is the object of the present invention to improve the protection structure so that the bipolar state of the transistor is more stable while the change to the bipolar state is still fast.